Mixed phase pigments having a rutile structure have long been known. By incorporating metal oxides such as NiO, Cr.sub.2 O.sub.3, CuO or MnO with Sb.sub.2 O.sub.5, Nb.sub.2 O.sub.5 or WO.sub.3 in the crystal matrix of titanium dioxide, it is possible to prepare mixed oxides with a rutile structure which have colors extending over wide ranges of the visible spectrum.
If the valency of the color producing metal ion incorporated in the structure is other than 4, another metal oxide with a different valency is also incorporated in the crystal lattice to correct the valency so that the metal oxides incorporated in the lattice satisfy the following condition: ##EQU1##
It follows that a and b conform to the equation EQU a.multidot.n+b.multidot.m=4(a+b)
wherein
A is a metal capable of being incorporated, e.g. Cr.sup.3+, Ni.sup.2+, Cu.sup.2+, Mn.sup.3+ ;
B is another metal capable of being incorporated, e.g., Sb.sup.5+, Nb.sup.5+, W.sup.6+ ;
O is oxygen atom;
n is the valency of the incorporated metal A, and is 2 or 3;
m is the valency of the incorporated metal B, and is 5 or 6; and
a and b are small whole numbers.
These proportions are generally fairly closely observed but rutile mixed phase pigments are known in which the proportions deviate more widely from this idealized composition.
Rutile mixed phase pigments of nickel and chromium have achieved considerable technical importance. When nickel oxide and chromium oxide are incorporated in TiO.sub.2 as color producing oxides, their valencies are compensated by the incorporation of other metal oxides of higher valency, particularly oxides of antimony but also of niobium and/or tungsten (U.S. Pat. Nos. 1,945,809, 2,251,829, 2,257,278 and 3,832,205).
Pigments of this kind are prepared by annealing anatase and/or hydrates of titanium dioxide with metal oxides which are capable of being incorporated into the crystal lattice or with precursors of such metal oxides (U.S. Pat. No. 3,022,186).
The products of annealing are usually suspended in water and the pigments are then obtained in their final form normally by grinding (preferably wet grinding) to the required particle size followed by washing, drying and grinding to break up any agglomerates.
The pH of the products obtained (according to DIN 53 200) varies from slightly alkaline to slightly acid according to the particular method used for working up.
In lacquers hardened by acid catalysis, rutile mixed phase pigments in the untreated form tend to undergo substantial flocculation which, as is well known, leads to a color change.
Surface-treatments of rutile mixed phase pigments using inorganic agents are known from the literature. For example, according to German Offenlegungsschrift No. 2 936 746, the untreated pigments are first treated with a surface active agent and then a single layer of metal hydroxide of the elements Ti, Zr, Sn or Al is applied.
Although rutile mixed phase pigments which have been given a single layered inorganic surface-treatment show improved dispersibility, their tendency to flocculate in lacquers hardened with the aid of acid catalysts is not substantially improved.